Screw type extruders comprise a cylinder in which an extruder screw is rotatable. It is used for extruding plastic material, rubber and other thermoplasts and elastomers and other plastic masses. Screw type extruders have been used successfully for many decades for extruding such material. Many different screw constructions have been developed in order to obtain good homogenization, disintegration and working of the materials according to the requirements of the different kinds of material. With very long extruders or with several extruders arranged in series, materials which are difficult to work can be broken up and homogenized in the desired manner particularly when the extruder is heated or cooled in selected places. However, the large number of existing extruder screw constructions indicates that working with screw type extruders is not without problems. If the working of the material takes places too rapidly and in too short a distance, the material being worked is heated to such extent that it can lose important characteristics or can be rendered completely worthless. If the working takes place too slowly, the material is often not fully worked when it leaves the extruder.
These problems are rendered more pressing by the fact that in general, worked and disintegrated material lies in the extruder together with unworked material which is fed with the worked material to the nozzle of the extruder. The worked and therefore more viscous material envelops the unworked and therefore less viscous material, thereby bestowing on it excellent sliding characteristics and leading to the possibility that the unworked material is forwarded to the screw tip.
It has been known very early that the working of an extruder screw can be considerably improved by providing in the web of the extruder screw openings which lead from the pressure side to the lee side of the web (DT-PS German Pat. No. 402879). It is now known that this improvement in the working of the extruder screw results from the fact that the frequency of occurrence of unworked material portions is much greater on the lee side than on the pressure side of the screw web. If the worked material on the pressure side of the screw web is permitted to pass through openings in the web and thus divide the stream of material in a screw passage into two parts, the unworked portion is thereby brought onto the pressure side of the screw web flank where it is more strongly worked than on the lee side.
However, for a long time the experts have not been able to develop this knowledge further. Instead, they have chosen other routes to separate the more viscous portion from the less viscous portion and to achieve in this manner that all portions are subjected to working in a particular short section of the extruder screw. DT-PS German Pat. No. 910218 shows a screw type press in which the screw is single threaded in an entrance portion, is double threaded in a working zone and is again single threaded in a discharge zone. Here, there is provided in the two thread part a screw web back over which the material must flow in passing from the first passage to the second passage. Through this screw web back, not only is the less viscous unworked material held back but also the round form of this screw web back performs a rolling mill operation on the material before it can enter the second screw passage. At the end of this rolling process, the material is sufficiently rolled out that it can pass between the round screw web back and the inner wall of the cylinder. This so far rolled material is later carried along by the following screw flank which is not rounded. With this known extruder screw the unworked material is pushed under relatively low pressure in a direction parallel to the round screw web back over which the worked fused material is led forwardly to the ejection zone thus in a forward direction and is repeatedly subjected to the rolling mill working of this round back. For plastic material only one two screw part is needed; for rubber it is necessary to have at least two such parts. With this screw extruder surprisingly good results are obtained. The material is received in a well homogenized state. The temperature of the extrusion is wholly uniform.
An identical arrangement of the screw web is disclosed in German Pat. No. 1,207,074. However, here the screw web back is no longer round but is formed the same as the other screw webs. For plastic, this machine works in the same manner by separating the more viscous and less viscous parts but without the rolling mill working of the round back.
Despite the good results obtained with both of these extruder screw designs, they are not suitable for all materials. As all of the material must cross the screw web back one or more times, the amount of energy required to drive the screw is not unimportant. The temperature division along the screw is not uniform. The screw geometry does not exclude the possibility of there being places where the material remains for a longer time and is damaged or indeed destroyed by the high temperature.
Only after the present working processes became known did the technique again seize upon the idea of dividing the material found in the screw passage. The DT-AS No. 1302096 shows one possibility of this by two single thread helical webs of like pitch overlapping one another so that there is a short section of a two thread screw between the single thread sections. Also, in this manner is it achieved that the material from the lee side in the two thread portion is so led that it lies on the pressure side of the flank of the screw web in the following single thread part. By the DT-AS No. 1302096 this is achieved without any inherent resistance. It is otherwise with DT-PS Pat. No. 1816440. Here, the web pieces at the places where the material stream in the screw passage is divided into two passages are transverse to the direction of the screw passage. These transversely extending web pieces where extensive unworked portions of the material are present represent a very strong resistance which not only greatly increases the power required to drive the machine but also severely stresses the material to be worked.
This construction uses similar working means which have proved successful as a shearing part at the end of a preferably single thread screw. This shearing part is multiple threaded and its screw webs are formed as forwarding webs. Between the screw webs lie screw passages which are drawn through from each essentially axial web and whose profile forms a wedge space with the extruder cylinder which envelops the screw. Such a shearing part is known through DT-OS No. 1729364. Here, preferably in a single thread screw the stream of material to be worked at the end of the working zone is divided into a plurality of stream parts and each stream part is forced over a transverse web in the screw passage. In this manner the already worked material is subjected to outstanding shearing and thereby homogenization. The division into several screw passages and the resulting simultaneous leading of the material over a transverse web in each of the passages serves as the working means for carrying out this process. Such working means which are very good for the shearing of already worked material are not however so well suited for subjecting less viscous unworked material to a working process. This is indeed also possible but however the material to be worked is subjected to high warming particularly when the transverse webs in the screw passage are short.